This invention relates to tire curing molds or matrices and in particular, to a new and improved adjustable mold particularly suited for use in retreading operations.
The general operation for producing a tire includes placing the tread material on the periphery of the casing, positioning the tire in a mold, and applying heat to cure the tread material, bonding the tread to the casing and forming the desired pattern.
Two types of molds are in general use today. One type uses two rigid annular sections, an upper section and a lower section which are joined together with the tire therebetween. In the other type, a plurality of rigid segments are brought into abutting engagement with each other to form the mold about the tire. A wide variety of mechanisms, both simple and complex, are in use for manipulating the mold pieces and the tire. All of these provide a fixed diameter for the mold.
In a different mode of operation, tires are also retreaded by utilizing a pre-cured tread rubber which is provided to the tire retreader in long strips. A piece of the cured tread rubber is cut to length and applied to the tire casing and is then vulcanized in place in a curing envelope. No curing mold is used with this type of operation. The pre-cured tread material is expensive.
Another approach to tire retreading is shown in the U.S. Pat. No. 2,014,010. In this arrangement, the retread material is positioned on the casing and a flexible band carrying the desired mold pattern is stretched over the retread material, with steam or air under pressure acting on the exterior of the non uniform band for pressing the mold into the uncured rubber. This assembly is then subjected to heat for vulcanizing the retread material in place. This system never became practical because of poor heat transfer characteristics and because the unsupported band had a very short life.
Tires of the same nominal size actually vary considerably in diameter, typically over a range of .+-. 1/2 of an inch outside diameter. When such tires are being retreaded in the presently used molds, either the two piece upper and lower mold or the multi segment mold, the tire is deformed to the fixed diameter of the rigid mold. This is not satisfactory because the tire is not cured in its most favorable condition unless it happens to exactly fit the mold with the beads in the same position they will be in when the tire is mounted on its rim. Tires that are cured in other than their optimum position do not perform as well as properly cured tires. Tires that are distorted when cured will run hotter, having an increased chance of failure and will not deliver the same wear as the tire cured in a mold of the precise size of the tire. One solution for this problem has been to have two or three molds of slightly different diameter available for use with each nominal tire size. However this requires a substantial increase in the initial cost of the equipment. Also, where automatic machinery is being utilized, a substantial amount of time and labor is required for changing molds.
The problem has become more serious in recent years with the advent of the radial tire, since the diameter of the radial tire is not readily reduced to fit a mold in the same manner as is the case with the bias tire. That is, a retread for a radial tire must be cured at a diameter very closely related to the belt diameter of the particular tire being handled.
It is an object of the present invention to provide a new and improved tire curing mold which has the advantages of the presently used rigid molds, while being adjustable for handling tires of varying diameter. A further object is to provide such an adjustable mold which can maintain tread continuity and which can have the heat transfer characteristics and long life of the conventional rigid mold of fixed diameter. Other objects, advantages, features and results will more fully appear in the course of the following description.